Generally, titanium (or titanium alloy) implants are subjected to a variety of surface treatments to improve biocompatibility thereof following lathe processing and milling. As examples of such surface treatment methods, mention may be made of etching in acidic/alkaline solutions, particle blasting, plasma spray, thermal oxidation, sol-gel induced coating using bioactive materials such as hydroxyapatite, bioglass and bioceramics, physical/chemical vapor deposition, Ion Implantation or Plasma Source Ion Implantation, electrochemical anodic oxidation and applied techniques using any combination thereof.
As examples of surface treatment methods using the electrochemical anodic oxidation among those methods, there are known a method for preparing an oxide film using a mixed solution of sulfuric acid and hydrochloric acid, or sulfuric acid and phosphoric acid, or phosphoric acid and oxalic acid (German Patent No. 2,216,432, Japanese Patent Publication Laid-Open No. Hei 02-194,195, Swedish Patent No. 1999-01973), a method for preparing an oxide film containing calcium and phosphorus (U.S. Pat. No. 5,478,237), a method involving forming an anodized film, followed by heat treatment (U.S. Pat. No. 5,354,390), a method involving forming calcium-phosphate using anodic oxidation, followed by heat of hydration treatment so as to prepare hydroxyapatite (U.S. Pat. No. 5,354,390), a method for preparing dicalcium phosphate anhydrous (DCPA, CaHPO), tricalcium phosphate (alpha-TCP), amorphous calcium phosphate (ACP) and dicalcium phosphate dihydrate (DCPD) (U.S. Pat. No. 5,997,62), using anodic oxidation, and a method for preparing a titanium oxide film by anodic oxidation (EP Patent Publication No. 0 676 179).
However, implants in which titanium or titanium alloy was coated with the above-mentioned calcium-phosphate or hydroxyapatite, have suffered from delamination of coated materials, or biodegradation and resorption due to biological actions at interfaces between an implant body and coating materials or inside coating materials, thus causing chronic inflammation of bone tissue in the vicinity of the implant, and thereby prolonged use thereof may result in continuous drop of a success rate. Further, the thicker the oxide film is, the lower the mechanical strength of the oxide film is, and the oxide film may be delaminated into bone tissue from the interface between the implant and bone tissue.